I mean, yes. But there’s no 5mm of air between tritium you drink and your stomach.

You're probably thinking of getting a speck of a radioactive metal (like Uranium or Plutonium) in your lungs. That gets stuck there, the body has no mechanisms to eliminate it, and it will keep emitting gamma rays all your life.

Tritium is very different. It is an isotope of Hydrogen, and a water molecule containing one atom of Tritium is fairly indistinguishable from any other water molecule. The most important thing is that it has no reason to stick to any other Tritium from that glass that you drank. It will spread throughout your body. Like all other water molecules, it will probably be out of your body in a mater of days. Tritium's half life is 12 years, so the vast, super vast majority of all the Tritium you'd drink with that glass of water will be long gone before it has a chance to decay.

But even if some of these atoms decay, their decay is a beta particle (an electron, and if you really care one antineutrino as well, although that antineutrino will probably travel through a few galaxies before it interacts with anything). The beta particle is not very damaging (compare that with the gamma rays emitted by Plutonium). For sure a single one is not damaging at all. Now, in contrast with the speck of the radioactive metal above, this beta particle will not be followed by others like it, because the Tritium that emitted it doesn't have any other Tritiums next to it.

Hmm, if people were worried about ingestion I was certain it had to be alpha particles. I stand corrected.

I’m not sure how much the half life matters a lot if you ingest a lot. To be fair, kinda unlikely from drinking water, but might be if a fish has been gathering Tritium it’s whole life (but then, it’s probably already doing that now for much larger quantities).

Either way, I suppose the 3 X-rays I had today completely obviate any relevance of the tritium.

> might be if a fish has been gathering Tritium it’s whole life

Unless fish have some mechanism for selectively keeping tritium (which might be very useful) or don't replace their water, there's no way for a fish to "[gather] Tritium its whole life".

> But there’s no 5mm of air between tritium you drink and your stomach.

Correct; it's in liquid, which blocks its radiation even more effectively.

The mucous layer in the stomach and intestines will stop it cold. It can't even get past the layer of dead cells that make up our skin.

Notwithstanding your qualifications, this reply is a bit off the mark. The tritium in that vial is indeed no danger, the beta decay of the tritium won't get past the vial itself. Tritium ingested is indeed a danger, however, because once in the body, the water molecules that carry the tritium can and will go anywhere in the body. Most will be excreted in the urine, but some will some will end up in cell nuclei, where their low energy beta decay can and does do real damage to DNA.

Note that I am not arguing that the tritium releases proposed in Japan are dangerous. They are far below any theshold for risk that should concern us. But that's because the concentration is so low, not because ingested tritiated water isn't potentially dangerous.

Thank you. This was the type of answer I was looking for.